The present application claims priority to Japanese Application(s) No(s). P2000-263497 filed Aug. 31, 2000, which application(s) is/are incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
The present invention relates to an optical recording medium and a method of manufacturing the optical recording medium, and more particularly to an optical recording medium which is suitable for application to the case of use in a near field state in which a distance between an optical lens and the optical recording medium is caused to approximate to 200 nm or less and a method of manufacturing the optical recording medium.
2. Description of the Related Art
As an optical recording medium, such as an optical disc, there are so-called ROM type optical discs dedicated for reproduction with a pit and a tracking groove formed preliminarily on a substrate, and so-called RAM type optical discs for recording and reproduction such as a phase change type optical disc and a magneto-optic disc with an information recording layer formed on a substrate with the above-mentioned pit and tracking groove formed.
The phase change type optical disc performs recording of information by changing the phase of a recording layer between an amorphous state and a crystalline state by irradiation of laser light, and reproducing of the information by optically detecting a change in the refractive index thereof as a change in reflectance.
Moreover, the magneto-optic disc performs recording of information by changing the magnetization state of a recording layer by application of an external magnetic field or by laser irradiation, and reproduction of the information by rotating a polarizing angle of the reproduction laser beam according to the magnetization state by the magneto-optic effect, such as the Kerr effect, and detecting the rotation.
These optical discs have a film formation layer of, for example, a recording layer, a reflection film, a dielectric film, or the like on a light transmissible substrate such that the information can be recorded in or reproduced from the recording layer by having a laser beam thereupon from the substrate side as mentioned above.
The recording density of an optical disc, in general, depends on the laser spot size of a light source so that the recording density can be made higher with a smaller laser spot size. The laser spot size is proportional to xcex/NA (xcex: laser light beam wavelength, NA: numerical aperture of an objective lens). Therefore in order to increase the recording density of an optical recording medium, it is required to have a short laser beam wavelength, and a high NA.
As a method for realizing a high NA, recently, a near field optical disc with a 200 nm or less distance between an optical disc and an optical lends has been studied elaborately. The present inventors have reported their study of achieving an NA greater than 1 optical parameter using a solid immersion lens (SIL) (for example, I. Ichimura, K. Kishima, K. Osato, K. Yamamoto, Y. Kuroda, K. Saito, xe2x80x9cNear-Field Phase-Change Optical Recording of 1.36-Numerical-Aperturexe2x80x9d, J. Appl. Phys. Vol. 39, 962-967 (2000)).
In the case of achieving a high NA in an optical system, a problem of a large comatic aberration is generated. The comatic aberration is proportional to (skew angle)xc3x97(NA)3xc3x97(distance of transmission of a laser light beam in an optical disc) (the skew angle denotes the tilt angle with respect to the optical axis of an optical disc).
As mentioned above, according to conventional optical discs, since a laser light beam is irradiated to a recording layer from the substrate side, the substrate should be thin in order to reduce the comatic aberration. As for the substrate of an optical disc, a plastic injection molding substrate is used in large numbers, and thus it is problematic to make the substrate thinner with high accuracy in terms of production as well as strength.
On the other hand, there has been spread a recording and/or reproducing method for considerably reducing a distance to the recording layer of a laser beam by employing a recording or reproducing manner in which the laser beam irradiation during recording or reproduction is carried out on the protective film side formed on the recording layer on the opposite side to the substrate side of an optical disc. In the case of this method, since the comatic aberration can be reduced, it is preferable for achieving a high NA.
In the near field optical disc device in which a distance between an optical system such as an optical lens and an optical disc is caused to approximate to 200 nm or less, if a projection such as a convex defect is present on the surface of the optical disc, the optical system or the surface of the optical disc might be damaged. In the optical disc to be used for a near field, accordingly, the surface is to be flattened with high precision. In particular, no projection on the surface is strictly required.
Moreover, since the surface reflection can be reduced from a specific layer, or the multiple interference with respect to a specific layer can be adjusted by forming a predetermined protection layer consisting of a dielectric material on the surface of an optical disc used in the near field, the MTF (Modulation Transfer Function) can be improved. (I. Ichimura, K. Kishima, K. Osato, K. Yamamoto, Y. Kuroda, K. Saito, xe2x80x9cNear-Field Phase-Change Optical Recording of 1.36-Numerical-Aperturexe2x80x9d, J. Appl. Phys. Vol. 39, 962-967 (2000).)
The optical disc has a recording manner such as groove recording in which information is recorded on a recording layer in a groove for tracking, land recording in which information is recorded in a recording layer on a land between adjacent grooves or land-groove type recording in which information is recorded in both recording layers of the groove and the land. In the case of the land-groove recording, a recording density can be increased.
For example, in the case in which such land-groove recording is to be carried out, a film formation layer having the recording layer is formed along a fine concave-convex surface such as a pit or a groove which is formed on the substrate, that is, such that a concave-convex surface conforming to the shape of the surface of the concave-convex portion is generated. In this case, if the protective layer for improving the MTF, for example, is formed by sputtering as in the conventional method, the surface concave-convex portion cannot be relieved by the protective layer. In other words, a concave-convex portion is generated on the surface of the protective layer by the land and the groove, for example, so that the distance between the optical system and the surface of the optical disc is varied on the land and the groove. Consequently, their optical bonding becomes nonuniform so that a predetermined optical characteristic to be desired cannot be obtained.
For the disadvantage, the present inventors have proposed a flat layer to be formed by a spin coat method, capable of polishing the surface in the official gazettes of Japanese Patent Application Nos. 11-186842, 11-19674, and 2000-203967, or the like.
However, in the case of forming a flat layer by the spin coat method, the film shape thereof can be affected easily by the physical property value of the base material. In the case the surface tension of the base material is lower than the surface tension of the liquid for the spin coat, the liquid is repelled so that a flat layer with an even film thickness cannot be formed in some cases. In particular, in the case the flat layer thickness is thin, even if it is not repelled in the state immediately after the application by the spin coat, the liquid may be repelled in a subsequent baking step. Furthermore, even in the case it is not repelled, the film thickness may be uneven.
In the optical recording medium, particularly, the optical disc, the shape thereof is equivalent to that of a CD (Compact Disc) and is set to be a circular shape having a central hole in many cases.
In the optical disc, moreover, the film formation layer including the recording layer contains a material layer which comes in contact with outside air and is thereby corroded easily. During the formation of the film formation layer, for example, the sputtering, therefore, a non-formation region in which the film formation layer is not formed is provided in the outer peripheral edge portion of the optical disc and the vicinity of the central hole, and the outer peripheral side edge of the film formation layer based on the non-formation region and the central side edge are covered so that a protective film or cover layer formed of an ultraviolet thermosetting resin or the like is coated through spin coating or the like.
Also in the optical recording medium to be used in the near field, as shown in the schematic sectional view of FIG. 12, a light transmission flattening film 4 is formed to cover the outer peripheral side edge of the film formation layer 3 and the central side edge over the film formation layer 3 on the substrate 1 where the fine concave-convex portion 2 is formed.
In the optical recording medium to be used in the near field, however, the light transmission flattening film 4 is formed as a very thin film to have a thickness of 400 nm or less or 100 nm or less. Consequently, even if the light transmission flattening film 4 is coated by the spin coating, it is impossible to disregard the influence of a difference-in-height portion 10 formed by the central side edge of the film formation layer 3 and the outer peripheral side edge on the surface of the light transmission flattening film 4 which has not made troubles in the conventional optical disc.
Furthermore, since the central side edge of the film formation layer 3 and the outer peripheral side edge, that is, the end face of particularly the recording layer as exposed to the side portion of the difference-in-height portion 10 has a surface tension smaller than that of the flat film, the flat film spin coat liquid can easily be repelled at the difference-in-height portion 10. In particular, in the case the flat film thickness is substantially the same as the height of the difference-in-height portion 10, furthermore, in the case it is lower than that, the spin coat liquid is repelled further remarkably so that not only deterioration of the optical recording medium reliability and deterioration of the yield, but also radial thickness irregularity called striation is generated disturbing the surface flatness of the flat film, and thus a damage problem arises in the above-mentioned optical system.
As described above, moreover, in the case in which a blue laser beam is used by a reduction in a wavelength of the irradiated laser beam in order to increase the recording density, it is necessary to select a material having a low light absorption rate for the blue wavelength light in order to obtain excellent reproducing characteristics or recording and reproducing characteristics in the light transmission flattening film.
As a light transmission flattening film having a low absorption rate for the blue wavelength light, an inorganic material is preferable. In order to obtain an excellent flattening property, moreover, spin coating is preferable for a method of coating the light transmission flattening film. Conventionally, the coating solution is repelled. Consequently, there has been a problem in that reliability, yield and the like are deteriorated.
It is an object of the present invention to provide an optical recording medium having excellent reproducing characteristics or recording and reproducing characteristics and capable of enhancing reliability and yield, and a method of manufacturing the optical recording medium.
The present invention provides an optical recording medium for carrying out at least one of record and reproduction of information by light irradiation, comprising a substrate having a concave-convex portion formed on a main surface on a side where the light irradiation is to be carried out, a film formation layer provided with a concave-convex surface corresponding to the concave-convex portion which includes at least a recording layer over the main surface of the substrate, and a light transmission flattening film having a transmittance to the irradiated light which is formed on the film formation layer through a hydrophilic material film, wherein the concave-convex surface on a surface of the film formation layer is filled with the light transmission flattening film and is thus flattened.
In the present invention, the hydrophilic material designates a material layer having a surface tension of 40 [dyne/cm] or more.
Moreover, the present invention provides a method of manufacturing an optical recording medium for carrying out at least one of record and reproduction of information by light irradiation, comprising the steps of manufacturing a substrate having a concave-convex portion formed on a main surface on a side where the light irradiation is to be carried out, forming a film formation layer having at least a recording layer, forming a light transmission flattening film having a transmittance to the irradiated light and filling in a concave-convex surface generated on a surface of the film formation layer to flatten the surface, and forming a hydrophilic material film on a surface where the light transmission flattening film is to be formed before the step of forming a light transmission flattening film.
As mentioned above, according to the optical recording medium of the present invention, by forming a hydrophilic material film on the light transmissible flat film formation surface, in particular, on the surface with the bonding strength thereof being important, peel-off of the attached light transmissible flat film, and thickness unevenness can be avoided so as to improve the reliability.
Moreover, according to the production method of the present invention, by forming a hydrophilic material film, and thereafter forming a light transmissible flat film at the time of the light transmissible flat film, the phenomenon of repelling of the flat film formation material liquid also at the film formation layer rim part gap at the time of formation of the light transmissible flat film can be avoided.
According to the present invention, the light transmission flattening film formed of an inorganic material having a low absorption rate for a blue light having a short wavelength can be formed with an excellent surface property and a high adhesion strength by spin coating.